1. Field of the Invention
The present invention relates to an electromagnetic drive circuit to be used for driving a pendulum or the like.
2. Description of the Prior Art
An example of a drive circuit for detecting and driving the pendulum of a clock with a single coil is shown in FIG. 8. The operations where a dipole permanent magnet M, as shown in FIG. 9, is to be driven by that drive circuit will be described in the following. As the permanent magnet M moves in the direction of the arrow sequentially to the positions a to e of FIG. 9, voltages, as shown in FIG. 10, are induced in a coil L.sub.2. Specifically, the induced voltage takes its maximum at the position c and has smaller amplitudes between the positions a and b and between d and e.
This induced voltage is generated at a terminal P of FIG. 8. When the induced voltage exceeds a reference voltage v.sub.r of FIG. 11A, a transistor T.sub.2 of FIG. 8 is turned off whereas a transistor T.sub.1 is turned on so that a drive current flows in the coil L.sub.2. The ON time t of the transistor T.sub.1 is determined by the time constant of a capacitor C and a resistor R.sub.1.
In order to drive the magnet efficiently, it is preferable to drive it at the maximal point (as indicated at c in FIG. 10) of the induced voltage. In order to effect the drive at that timing, the reference voltage v.sub.r and a drive time are properly set.
In case the pendulum is driven, the amplitude of the induced voltage will fluctuate depending upon the swing angle of the pendulum. If the reference voltage v.sub.r is set at a level of FIG. 11A, for example, the induced voltage may exceed the reference voltage other than at its maximal point as the swing angle increases so that the amplitude of the induced voltage augments, as shown in FIG. 11B. This excess may invite a malfunction.
If the reference voltage is set at a high level so as to prevent the malfunction, the induced voltage may fail to exceed the reference voltage and drive the magnet in case the swing angle is small or in case the pendulum has a long period.
This makes it necessary to change the reference voltage each time in accordance with the swing angle and period of the pendulum.
A detailed description of the prior art circuit constitution is given above, but the biggest defect thereof is the incapability of integrating the circuit constitution.